# Re: Need help on Functions Dependency problems, Urgent!!!

Date: 11 Nov 2002 12:42:06 +0100

Message-ID: <3dcf978e$1_at_news.uia.ac.be>

lordsofring wrote:

>I'm a newbie on function dependency problem, currently working on the

*>following questions, but havn't got a clue. Anyone can solve this will
**>be highly apprieciated.
*

Of course, but if we give you the answers you won't learn anything. So tell us what you don't understand and we will try to give you hints. For now I will give some very short hints:

*>
*

>F = {A->B, C->D, AC->E, B->E, CE->A, BD->C} is a set of function

*>dependencies on S (ABCDEF)
**>
**>1. Find all the candidate keys of S regarding to F.
*

Hint1: There is a rule that you can use to find smaller superkeys:

If K is a superkey and X->A a fun. dep. that holds (with X a set of attributes and A single attribute) then you can remove A from K if X is a subset of K.

Hint2: ABCDEF is the mother of all (super)keys

>2. Find a cover for F+|ABCD, F+ is the closure of F.

Hint: Just do what it says, determine F+|ABCD and then find a minimal cover for it. But try to be smart about it because it may otherwise be a lot of work.

>3. Find a BCNG and loss less join decomopsetion for S. explain the

*>procedure
*

Depends a bit on what your instructor has told you. The usual way:
- determine minimal cover

- split off dependencies that violate BCNF
Another possiblity:

- determine minimal cover

- give every fd its own relation

>4. Does the BCNF and loss less join decomposition in (3) preserve the

*>set F of fd's?
*

Hint: the definition of preserving F in decomposition D is that all fd's in F follow from the restrictions of F+ to the relations in D.

Good luck,

- Jan Hidders